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North Atlantic Hurricanes
Volume 10, Number 31: 1 August 2007

Hurricanes are getting stronger ... at least that's the word from Al Gore, who made this factual-sounding declaration in his 21 March 2007 testimony before the United States Senate's Environment & Public Works Committee. But is this contention true? And if so - or if not - why?

Interestingly, both of these questions have recently been answered by Latif et al. (2007), who began their study of the subject by analyzing the 1851-2005 history of Accumulated Cyclone Energy or ACE Index for the Atlantic basin, which parameter, in their words, "takes into account the number, strength and duration of all tropical storms in a season," after which they "analyzed the results of an atmospheric general circulation model forced by the history of observed global monthly sea surface temperatures for the period 1870-2003."

With respect to the first part of their study, Latif et al. report that "the ACE Index shows pronounced multidecadal variability, with enhanced tropical storm activity during the 1890s, 1950s and at present, and mostly reduced activity in between, but no sustained long-term trend [our italics]," while with respect to the second part of their study, they report that "a clear warming trend is seen in the tropical North Atlantic sea surface temperature," but they found that this warming trend "does not seem to influence the tropical storm activity [our italics]."

This state of affairs seemed puzzling at first, because a warming of the tropical North Atlantic is known to reduce vertical wind shear there and thus promote the development of tropical storms. However, Latif et al.'s modeling work revealed that a warming of the tropical Pacific enhances the vertical wind shear over the Atlantic, as does a warming of the tropical Indian Ocean. Consequently, they learned, as they describe it, that "the response of the vertical wind shear over the tropical Atlantic to a warming of all three tropical oceans, as observed during the last decades, will depend on the warming of the Indo-Pacific relative to that of the tropical North Atlantic," and "apparently," as they continue, "the warming trends of the three tropical oceans cancel with respect to their effects on the vertical wind shear over the tropical North Atlantic, so that the tropical cyclone activity [has] remained rather stable and mostly within the range of the natural multidecadal variability."

Nevertheless, a striking exception to this general state of affairs occurred in 2005, when the researchers report that "the tropical North Atlantic warmed more rapidly than the Indo-Pacific," which reduced vertical wind shear over the North Atlantic, producing the most intense Atlantic hurricane season of the historical record. In contrast, they say that the summer and fall of 2006 were "characterized by El Niņo conditions in the Indo-Pacific, leading to a rather small temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans," and they say that "this explains the weak tropical storm activity [of that year]."

Clearly, the temperature/hurricane connection is nowhere near as "one-dimensional" as Al Gore makes it out to be. Warming alone does not imply that hurricanes are getting stronger. Rather, as Latif et al. describe it, "the future evolution of Atlantic tropical storm activity will critically depend on the warming of the tropical North Atlantic relative to that in the Indo-Pacific region [our italics]," and in this context they note that "changes in the meridianal overturning circulation and their effect on tropical Atlantic sea surface temperatures have to be considered," and that "changes in ENSO statistics in the tropical Pacific may become important."

The bottom line is that it's anyone's guess as to what would actually happen in the real world if the earth were to experience additional substantial warming. However, since the global temperature rise of the 20th-century (which climate alarmists like Gore contend has been unprecedented over the past few millennia) has not yet led to a sustained long-term trend in hurricane intensity (in either direction), it is our opinion that the next century of hurricane activity will probably not be much different from that of the past century.

Sherwood, Keith and Craig Idso

Latif, M., Keenlyside, N. and Bader, J. 2007. Tropical sea surface temperature, vertical wind shear, and hurricane development. Geophysical Research Letters 34: 10.1029/2006GL027969.